Electroplating

Introduction

Chromium on bathroom taps, gold on electrical connectors and copper filling the microscopic wiring channels in computer chips are all electroplated. Electroplating uses an external power supply to drive metal ions from solution onto an object made the cathode. The chemistry is simple Faraday's-law electrolysis, but producing a smooth, bright, strongly bonded film requires close control of current, transport and surface chemistry.

Core explanation

The cell. The object to be coated is the cathode. The electrolyte contains ions of the plating metal, usually with other salts to improve conductivity. At the cathode, metal ions are reduced:

Mⁿ⁺ + ne⁻ → M

At a soluble anode made of the same metal, the reverse reaction occurs, M → Mⁿ⁺ + ne⁻, keeping the ion concentration roughly constant, as in copper and nickel plating. With an inert anode , water is oxidised instead (2H₂O → O₂ + 4H⁺ + 4e⁻), so the metal ions are gradually depleted and must be replenished, as in hard chromium plating.

Faraday's law and efficiency. The mass deposited is

m = (I t M ÷ nF) × φ

where φ is the cathode current efficiency. Competing reactions, mainly hydrogen evolution, consume part of the current. Acid copper sulfate baths have efficiencies close to 100%, nickel about 95%, while chromium deposition from chromic acid baths is often only 10–25%, with most of the current producing hydrogen. Film thickness follows by dividing by density and plated area.

Current density and mass transport. At low current density, deposition is under activation control, and metal atoms have time to find favourable lattice sites, giving dense, fine-grained films. As current density approaches the limiting current, metal ions near the surface are depleted. Protruding points receive ions more easily than recesses, so they grow faster, producing rough, nodular or powdery "burnt" deposits and even dendrites. Hydrogen evolution also increases, which can cause pitting and hydrogen embrittlement of steel substrates. Platers therefore work at a fraction of the limiting current, and agitate the solution to thin the diffusion layer.

Current distribution. Current prefers the shortest electrolyte path, so edges and corners nearest the anode receive more than recesses. Throwing power describes how uniformly a bath coats a shaped object. It improves with higher electrolyte conductivity and with a steep increase of overpotential with current density, which evens out the local rates.

Additives. Small quantities of organic compounds alter film quality dramatically. Brighteners and grain refiners adsorb on the growing surface and increase nucleation, giving fine-grained, reflective deposits. Levelling agents adsorb preferentially on peaks, where transport is fastest, so recesses fill faster. In chip manufacture, carefully balanced accelerator and suppressor additives make copper fill trenches from the bottom up without voids.

Surface preparation. Adhesion depends on a clean, oxide-free substrate. Degreasing, rinsing and a brief acid dip precede plating, and a thin strike layer may be applied first to promote bonding.

Safety and environment. Some plating baths contain highly toxic species, such as cyanide complexes or hexavalent chromium, so industry uses enclosed tanks, extraction and effluent treatment, and increasingly substitutes safer chemistries, for example trivalent chromium baths.

Formulae

m = φ I t M ÷ (nF); thickness d = m ÷ (ρA); φ = charge used for metal deposition ÷ total charge passed.

Step-by-step reasoning

To calculate a plating thickness:

1. Find total charge Q = I t. 2. Multiply by the current efficiency to get charge used for metal. 3. Divide by nF to find moles of metal, then multiply by M for mass. 4. Divide mass by density to get volume. 5. Divide volume by area to get thickness.

Visual explanation

Picture a cross-section of a plated screw thread. At low current the coating is an even band; at high current it bulges on the crests and is thin in the grooves. Adding a levelling agent shows the grooves filling up while the crests grow slowly.

Real-world analogy

Plating is like spray-painting a sculpture. Stand close with high pressure and the nearest bumps get thick, drippy paint while hollows stay bare. Stand further back and spray gently and the coat becomes thinner but far more even.

Real-world example

Decorative "chrome" on taps is typically a thick layer of bright nickel under a very thin chromium top layer only a fraction of a micrometre thick. The nickel provides smoothness and corrosion resistance; the chromium adds hardness, tarnish resistance and the bluish shine.

Why?

Why is agitation used in plating baths? Stirring or moving the work thins the diffusion layer, raising the limiting current. This allows faster plating at the same quality and reduces local depletion that would otherwise cause rough deposits.

Common misconception

"Doubling the current always doubles the plating rate." It does so only while current efficiency stays constant. Near the limiting current, extra current goes largely into hydrogen evolution and produces poor-quality deposits.

Worked example

Question: A part of area 50 cm² is nickel-plated at 1.0 A for 30 min with 95% efficiency. Find the thickness. (Ni: M = 58.7 g/mol, n = 2, ρ = 8.9 g/cm³)

Reasoning: Q = 1.0 × 1800 = 1800 C; charge for nickel = 0.95 × 1800 = 1710 C. Moles of Ni = 1710 ÷ (2 × 96 485) ≈ 8.86 × 10⁻³ mol. Mass ≈ 0.520 g. Volume = 0.520 ÷ 8.9 ≈ 0.0584 cm³. Thickness = 0.0584 ÷ 50 ≈ 1.17 × 10⁻³ cm.

Answer: About 12 μm.

Quick check

1. What is the main competing reaction that lowers cathode current efficiency in aqueous plating baths? Answer: Reduction of water or hydrogen ions to hydrogen gas at the cathode.

Exam focus

Combine Faraday's law with current efficiency, density and area in multi-step calculations. Explain why high current densities give rough deposits in terms of mass transport and the limiting current, and distinguish soluble from inert anodes.

Advanced insight

Pulse plating, in which current is switched on and off or reversed in short bursts, lets ion concentrations recover near the surface between pulses. It allows high instantaneous current densities, which favour nucleation and fine grains, without the depletion that steady high currents would cause.

Summary

Electroplating reduces metal ions at a cathode to form a thin film, with soluble anodes replenishing ions or inert anodes oxidising water. Deposit mass follows Faraday's law adjusted for current efficiency. Film quality depends on current density relative to the limiting current, current distribution, agitation, additives and careful surface preparation.

Practice questions

1. Write the cathode and anode reactions for copper plating with a copper anode. Answer: Cathode: Cu²⁺ + 2e⁻ → Cu; anode: Cu → Cu²⁺ + 2e⁻. 2. A chromium bath has 20% efficiency. How much charge is used for chromium if 10 000 C is passed? Answer: 0.20 × 10 000 = 2000 C. 3. Why do high current densities produce rough, powdery deposits? Answer: Metal ions are depleted near the surface, so growth is transport-limited and favours protrusions, while hydrogen evolution increases. 4. What is the purpose of a levelling agent? Answer: It adsorbs mainly on high points, slowing deposition there so recesses fill and the surface becomes smoother.